1. Field of the Invention
This patent specification relates to measurements made in a borehole. More particularly, this patent specification relates to systems and methods for tool orientation measurements made in cased boreholes.
2. Background of the Invention
Traditionally, directional measurement devices used in tools designed to operate in open, uncased, boreholes have been based on a compass or other magnetometers and accelerometers. However, when operating inside a steel casing, such magnetic based measurements are not possible. Therefore, measuring the orientation of a borehole tool in a cased borehole environment has had difficulties. In addition, in some cases there are difficulties in open borehole measurements for oil wells close to north or south poles that have downward magnetic directions causing the compass to function incorrectly. Current solutions include the use of optical gyros and mechanical gyros. Unlike other navigation applications for gyros, logging tools can experience many rapid turns while traveling along the borehole in addition to the surrounding environment which can be hostile in terms of pressures and temperatures, etc. The errors of such types of gyros start to accumulate during the descent and in-situ calibrations using independent measurement are necessary in order to ensure the quality and the accuracy of such measurements.
Some inclinometry tools such as Schlumberger's General Purpose Inclinometry Tool (GPIT) combines magnetometers and accelerometers to measure the orientations of a borehole tool while logging. For many years there are industrial wide research efforts to solve this difficult cased borehole problem without success. Commercial attempts have been made to provide gryo-based orientation and steering capabilities while drilling, as well as gyro-based wireline logging tools. For example, see Halliburton's Evader® Cryo-While Drilling Service; and Geo-Guide ALC™ from Gryodata Inc. However, unlike airplane application the borehole tool will experience many turns while traveling up and down the borehole and therefore, even a gyro will be subjected to large error accumulations and generally requires independent in-situ calibration which itself is technically challenging in addition to the surrounding hostile environment.
Prior attempts to solve the cased hole tool orientation problem have been aimed at duplicating the open borehole tool direction measurements while tool is inside a cased borehole. Generally speaking, there are three angular unknowns, azimuth, inclination and rotation that need to be measured in order to uniquely determine the borehole tool orientations. The azimuthal angle with respect to the north-south direction requires a reference such as the North Pole and this is particularly challenging to measure while inside a cased borehole without a gyro like device because the steel casing interferes with the external magnetic fields.